The Method
The vast majority of presently used photographic processes depend on utilizing a film or paper substrate onto which a light sensitive silver based chemical compound has been coated along with other substances such as gelatin or dyes. A PSM can record a narrow spectral range, a wide spectral range, or several spectral ranges simultaneously. The final results are dependent on the changes occurring in the light sensitive (photosensitive) silver based layer(s) during exposure and subsequent development. The overall characteristics of any particular PSM, including such factors as spectral sensitivity, contrast, color balance, speed, density, tonal rendition, etc. are normally "locked in" during the manufacturing process, e.g. if a particular PSM is manufactured to produce or record only a high contrast image, little can normally be done to alter this basic characteristic using existing methods.
Whether the PSM is meant to depict final results in black and white, color, or otherwise, one of the most important factors dictating the suitability of a particular PSM for a particular purpose is contrast. While certain modifications to contrast characteristics are possible using methods such as those outlined below, these methods have severe limitations.
The present invention describes a means and apparatus for selectively altering the inherent contrast characteristics of the light sensitive silver layer(s) in a silver based PSM. In dealing with black and white photosensitive materials this can involve as few as one photosensitve silver layer. In the case of so-called "color" materials, it can involve several photosensitive layers coated one over the other on the same substrate and disposed in such a way that each layer will record only a particular region of the total spectrum (usually the red, blue and green "primary color" regions).
In one way or another, depending on the exact nature of the particular color PSM being utilized, the latent image recorded by each individual light sensitive silver layer is converted by subsequent processing steps into a color image corresponding to the spectral sensitivity of each individual layer and in proportion to the amount of exposure each layer received. Further processing steps usually remove all of the exposed or unexposed silver compounds leaving only the color dyes on the substrate.
Photographic materials are generally divided into basic categories: color, black and white, negatives, prints, slides, instant prints, etc. Common among these are photographic "prints" which are usually made by taking a picture with a camera on a special type of film which is chemically processed by a laboratory--first as a film "negative"--which is then used to make a reproduction on paper which is called a "print." Prints may be either black and white or color depending on the type of negative film used to take the original picture.
Another common way of taking pictures is by using a different type of film which results in a color slide. A picture is made with a camera and the exposed "film" is then chemically processed by a laboratory. The result is a piece of positive film commonly mounted in a cardboard frame called a "slide." To view slides they are most often projected onto a screen or wall.
By using special types of paper such labs can also make prints directly from "slides," but the results are often less than satisfactory. Prints made from slides can also be produced by using a special "internegative" process. In this case a special type of film is used to make a negative from the original slide, then the resulting "internegative" is used to make a paper prints.
Since the advent of practical photographic methods during the last century enormous progress has been made in improving the overall quality of photographic materials and processes. One of the problems still encountered is minimizing the losses in detail usually experienced when making reproductions from original negatives, transparencies, or other prints. When making reproduction with any photographic process it is desirable to utilize the least number of steps possible, (ideally one) since each additional step (generation) usually causes some detail to be lost or altered.
Some of the factors involved in the reproduction processes are: "resolution"--the ability of the reproduction material to depict the small detail recorded on the original film; "graininess"--this is the undesirable sand like appearance sometimes seen in prints or slides; in the case of color materials, "color balance"--the ability to accurately and faithfully reproduce the colors in the original picture; "contrast"--the visible difference between the lightest and the darkest areas in a picture. A color slide taken at the beach on a sunny day represents an example of a high contrast picture, that is the slide will show a large visible difference between the darkest and brightest areas in the picture. A similar slide taken on a rainy day would represent a low contrast picture; that is, the visible difference between the darkest and lightest areas in the picture would be considerably less than the difference seen in a high contrast slide.
Due to such commonly encountered lighting conditions the photographic record of such events often exhibit very high contrast ratios (1000:1); that is, the lightest area in a picture is 1,000 times lighter than the darkest area. Sometimes this difference can even be greater than 1000:1.
There are several methods commonly used to produce prints:
A. DIRECT POSITIVE PRINTS: In this process the prints are made directly from "transparent" originals, such as 35 mm color slides, which appear visually the same as the original scene (a positive image).
B. NEGATIVE PRINTS: Prints are made using film "negatives" to produce a positive "print" on paper or film.
C. INTERNEGATIVE PRINTS: With this method an intermediate "negative" is made from a positive original (a slide most commonly) and the "internegative" is then used to make a "print" on paper or film.
D. DYE TRANSFER PRINTS: A highly labor intensive and very expensive "manual" method requiring multiple steps to make a print.
Due to inherent differences between the characteristics of the photosensitive material (PSM) used to record original subject matter and the characteristics of the PSM used to reproduce copies of such originals, detail and other desirable image characteristics are often lost, degraded or altered.
To make reproductions from originals which have high contrast ratios, such as 1000:1, the PSM used to made the reproduction must have the ability to record such a high ratio; otherwise the detail or information container in the original will be lost or degraded during the reproduction process. Presently available photosensitive papers generally are capable of recording contrast ratios of less than 500:1. This means that reproductions made without intermediate steps on such materials will result in a significant loss of information. Reproductions produced by means of intermediate steps, such as internegatives, are able to record the contrast ratio of the original more closely, but they suffer from other deficiencies introduced by the multiple generation steps required.
Several techniques have been developed over the years to improve the inherent material limitations in making reproductions, but they too have limitations. Such techniques include: